350 LECTURE XXI. 



the protoplasm of the Fungi which produce them. So much for general guidance as 

 to the unorganised ferments and their differences from the exciting agents in alcoholic 

 and similar fermentations. 



Coming now to speak of the hitherto entirely unexplained dormant periods of 

 plants, in connection with what has been said with respect to ferments, it may appear 

 at first sight that no conceivable connection exists. I also by no means wish to 

 maintain that the view which I propose here is sufficiently proved, though I do 

 maintain that it opens up for us in the meantime a way to further investigation 

 of one of the most general phenomena of plant life. 



Even under the most favourable conditions of vegetation, dormant periods 

 occur in the course of the life of the plant. Under circumstances Avhen the plant 

 would be in a condition to grow most vigorously, because it is provided with reserve- 

 materials, and water and oxygen are at its disposal, and a sufficiently high tem- 

 perature might be expected to call forth the internal movements, every externally 

 perceptible vital motion nevertheless ceases, and it is only after some months of rest that 

 the growth commences anew, and this frequently under circumstances which appear 

 far less favourable — especially at a conspicuously lower temperature. This periodic 

 alternation of vegetative activity and rest, is in general so regulated that for a given 

 species of plant both occur at definite times of the year, leading to the inference that 

 the periodicity only depends upon the alternation of the seasons, and therefore chiefly 

 upon that of temperature and moisture. Without wishing to deny the co-operation 

 of these factors, a closer consideration shows however that this matter must depend 

 chiefly upon changes which take place in the resting plant, independently of external 

 influences, or only indirectly affected by them. To render the facts in question 

 intelligible to the reader, however, I will in the first place select from the enormous 

 mass of material a few well-known examples. 



The leaf-shoot and flowers contained in the bulb of the Crown Imperial com- 

 mence to grow vigorously in the spring time with us, even at the beginning or 

 middle of March, when the soil in which the bulb has passed the wdnter possesses a 

 temperature of 6-io° C. ; the leaf-shoots protrude forcibly from the cold earth to grow 

 vigorously in the but slightly warmer air. There would be but litde to surprise us 

 in this if we did not at the same time notice the fact that a new leaf-shoot is already 

 formed in embryo in the subterranean bulb in April and May : this shoot, however, 

 does not grow to any extent in the warm soil during the summer and autumn. On 

 the contrary, this favourable period of vegetation passes by, until at the end of the 

 winter an inconsiderable rise of temperature above the freezing-point suffices to in- 

 duce vigorous growth; and, as is well known, the same is the case with most 

 bulbous and tuberous plants, many of which, as the Meadow Saffron, possess two 

 active periods, the flowers developing in the late autumn and the foliage-leaves 

 belonging to them only in the next spring. The best known examples, however, 

 are afforded by the common potato and the kitchen onion. I have many times 

 attempted to induce the tubers and bulbs ripened in the autumn to put forth their 

 germinal shoots during November, December, and January, by laying them in moist, 

 warm, loose soil; but in the case of the potato as well as in that of the kitchen onion, 

 no trace of germination appeared. If, on the other hand, the attem.pt is repeated 

 in February, or still better in March, the germinal buds begin to grow vigorously 



